Quite a few flutists are modifying keywork for comfort using a variety of solutions. The only caveats that I have about this are to make certain that added weight from extensions isn’t a problem, that mechanically your instrument can support what you’d like to add, and that the problem of discomfort when playing isn’t better addressed by rethinking your body. Otherwise extensions can create other problems in the instrument or else in your hands. I only work with someone in the San Francisco Bay Area on extensions or modifications to a flute. People’s hands are very different and there’s no generic solution to modifications. And sometimes you have to go through a process of elimination to determine the best solution.